A wide variety of security devices for security documents, such as banknotes, traveller's cheques, or the like, have been previously proposed. Such security devices are provided in order to make falsification and counterfeiting of the security documents difficult. Amongst those considered to be effective for use in banknotes and like security documents are security devices which modify the characteristics of an impinging light beam. Such modification is generally detected by a photo-sensitive device which detects the light beam reflected from the security document and, by analyzing the characteristics of the reflected beam, verifies the authenticity of the security document. According to one known security document verification system, a diffractive device is applied to the surface of a banknote. The output of a laser diode is passed through a lens system to produce a beam of light which is deflected onto the diffractive device on the surface of the banknote. The diffractive device reflects the incident beam to thus generate a diffracted beam. One or more images are produced by projecting the diffracted beam onto a viewing screen. The characteristics of the diffractive device are chosen so that a preselected image is reflected onto the viewing screen, thus enabling the visual verification of the authenticity of the security document.
Such methods of verification, however, require the use of a complex system of dedicated laser beam sources, mirrors, lenses and projected image viewing screens. Accordingly, verification of the authenticity of security documents bearing such security devices is only possible by persons possessing the necessary verification equipment, and cannot be conveniently carried out by many retail outlet staff and like persons who wish to verify the authenticity of banknotes, cheques, traveller's cheques and such documents.
The aim of the invention is to provide a method of verifying the authenticity of a security document, which method is simple and convenient to use for a large number of persons who would wish to do so.
A further object of the invention is to provide a method of verifying the authenticity of a security document which does not require complex, dedicated verification equipment.
Another object of the invention is to provide a method of verifying the authenticity of a security document which ameliorates or overcomes problems of known methods of verifying the authenticity of security documents.